The present invention relates generally to turbines, particularly to land-based gas turbines for power generation, employing compressor air for cooling the buckets of the third turbine stage.
Steam cooling of hot gas path components of a gas turbine (for example, the buckets), has been proposed in the past and found viable in land-based power generating plants. While gas turbines are typically air cooled (for example, jet turbines employ compressor discharge air for cooling the hot gas path components), steam cooling is more efficient in that the losses associated with the use of steam as a coolant are not as great as the losses realized by extracting compressor bleed air for cooling. In land based gas turbines and especially those in combined cycle systems, steam cooling is particularly advantageous because the heat energy imparted to the steam as it cools the gas turbine components is recovered as useful work in driving the steam turbine in the combined cycle operation. However, while steam is preferred for cooling the first and second turbine stages, air is required to cool this third stage bucket, and (optionally) to purge the aft portion of the turbine rotor.
In accordance with this invention, air is extracted from the twelfth stage of the compressor and is carried through extraction piping outside the gas turbine, and then supplied through the turbine shell to the stage 3 nozzle. In order to reduce the cycle performance penalty of cooling the third stage bucket, relatively low pressure twelfth stage air is used. The traditional method of bringing air flow from the machine center line practiced by the assignor of this invention is not possible as the forward wheel cavities require high pressure air to drive their purge circuits. Even with steam cooling of the first two bucket stages, air is required to bathe the turbine wheels to control their temperature during transient and start-up operations. In other words, since the forward rotor cavities are filled with high pressure air, a new technique has to be devised for supplying low pressure compressor extraction air for air cooling the third stage. As a result, the bucket cooling air is supplied radially inwardly through the adjacent stator structure, i.e., the third stage nozzle, and then routed to the third stage bucket. In addition, access to relatively low pressure air also provides an optional air source for purge flow in the aft portion of the turbine rotor, with reduced cycle performance penalty. Thus, the invention seeks to introduce low pressure extraction air to the turbine rotor at a low temperature relative to the rotor for use in cooling the third stage bucket. The invention also provides at least an option to make use of the above mentioned air flow to purge the aft section of the turbine rotor, but this is not a preferred arrangement.
In accordance with the invention, a nozzle inducer system comprises a system of tubes carrying the compressor extraction air from the turbine shell through the nozzle airfoils and into the nozzle diaphragm. At the outer end, this tube system penetrates the turbine shell at twenty-two circumferential locations in the exemplary embodiment. Once inside the turbine shell, the piping is split into two conduits, thereby introducing air into forty-four nozzle vanes or airfoils. At the radially inner end of each nozzle vane, the air enters a passage in a diaphragm seal segment which directs the cooling air tangentially into a cavity surrounding the rotor. This passage is configured to accelerate the air in the direction of wheel rotation into this circumferential open area so as to substantially match the tangential velocity of the rotor spacer wheel located radially inwardly of the nozzle. The air is then fed into discrete sets of axial pipes which deliver the air to the shank passages of the stage three buckets. The air then flows radially outwardly through internal passages in the buckets and exits at the bucket tips, into the hot combustion gas path.
The air delivery system in accordance with the invention has several advantages. For example, the use of separate tubing for rotor delivery air minimizes heat transfer to the air from the hot nozzle airfoils. It also allows the use of lower pressure air to pressurize the outer side wall cavities and nozzle cooling circuits which reduces parasitic leakage, improving machine efficiency. In addition, due to the reduction in relative velocity between the rotor spacer wheel and the air, and the drop in air static temperature due to its tangential acceleration, a significantly lower temperature is available for bucket cooling compared to a design where air is simply fed radially into the rotor area.
Accordingly, in its broader aspects, the present invention relates to a land based gas turbine comprising a compressor, a combustor and at least three turbine stages fixed to a rotor, and specifically to an improvement which includes an air cooling circuit for the third turbine stage comprising an inlet into a third stage nozzle from a compressor for feeding cooling air from the compressor to the third stage nozzle; at least one passageway running substantially radially through each airfoil of the third stage nozzle and an associated diaphragm, into an annular space between the rotor and the diaphragm; and passageways communicating between the annular space and individual buckets of the third stage.
The present invention also relates to a method of cooling one stage of a gas turbine comprising a) extracting cooling air from a turbine compressor; b) supplying cooling air to a stationary nozzle adjacent the one stage of the gas turbine; c) establishing a path for the cooling air from the stationary nozzle to a plurality of buckets in the one turbine stage; and d) flowing the cooling air radially outwardly through the plurality of buckets and exhausting the cooling air from radially outer tips of the buckets.
Additional features of the subject invention will become apparent from the detailed description which follows.